June 30, 2011

Improved Membrane Water Transport

Robert Moore and Louis Madsen's groups at Virgina Tech claim to have done something quite interesting: increase the diffusivity of water in DuPont's NafionTM, the 50-year old membrane that is at the core of much current hydrogen fuel cell and electrolyzer work!

How did they do this? By stretching the NafionTM, using a draw ratio of up to 4:1.

Apparently, their data show that the water-conducting channels within the membrane become aligned during the stretching process. (See Figure 4 below.) In Figure 4.a, random water channels exist in the as-received membrane. In Figure 4.b, there is some alignment in the draw direction (blue arrow). Finally, in the at a 4:1 draw ratio shown in Figure 4.c, there is significant water channel alignment. Similar to nematic liquid crystals, the stretching does not change the water channel alignment or connectivity; this leads to anisotropic water transport.

Li et al Figure 4

The anisotropic nature of the water transport behavior is evident in the following Figure 2:

Li et al Figure 2

In the left figure, Figure 2.a, you can see how the diffusivity of water in the draw, or X direction (Dxx) is markedly higher than the diffusivity in the transverse directions (Dyy and Dzz). This relationship holds for all draw ratios tested, between 1:1 (as-received membrane) and 4:1.

Their data in Figure 2.a have been arithmetically averaged and plotted in Figure 2.b. This figure shows that the average water diffusivity curves for all the drawn membranes collapse to match that of the isotropic, as-received membrane.

What are the practical applications of this? Well, it may be possible to align membrane water passageways in the through-plane direction through the use of electric or magnetic field alignment methods used on liquid crystals, liquid crystal polymers and block copolymers. The resultant aligned membrane would have increased through-plane water (and proton) transport, resulting in improved PEM device (e.g. fuel cell) operation.